Optical transmission networks are favored in telecommunications due to the increase in bandwidth that they provide in comparison to traditional metal cable networks. Fiber optic channels are used as the transmission medium for optical signals, commonly optical pulse trains. As a transmitted signal propagates, it degrades due to a number of factors including absorption in the fiber channel.
Although amplifiers can be used to address the loss of signal amplitude, they do not aid in maintaining the hard edges of pulses. The hard edges of the pulses are used by optical elements in a clock recovery process. To ensure that signal degradation does not result in an unrecoverable signal, optical repeaters, also referred to as regenerators, are employed.
Conventional optical repeaters and regenerators are optical-electric-optical (OEO) devices that convert the received optical signal to its electrical counterpart, and then retransmit the signal in an optical form. Error correction can be employed after the conversion to the electrical domain, as can other functions.
In synchronous feed forward network architectures, such as Synchronous Optical Networks (SONET), repeaters rely upon the signal to maintain clocking synchronous to other network nodes. If one element in a network path fails, it should preferably do so in a manner that does not create problems for other elements. Similarly, a system that detects a failure should handle the failure in a manner that prevents the failure condition from being propagated to downstream nodes.
In many synchronous standards, elements such as repeaters have built-in reference clocks. Despite the presence of the reference clocks timing is recovered from the received signal and is fed-forward to the egress port for propagation to downstream network nodes. The use of the feed-forward timing is done to ensure timing consistency across the network.
In conventional synchronous systems such as SONET and Optical Transport Network (OTN) repeaters, failure conditions can occur and have catastrophic results. Fiber interruptions and line cuts, as well as other events causing an unplanned termination of service can result in a loss of signal. Because clocking information is embedded in the signal, a loss of signal eliminates the ability of the clock recovery circuitry to extract an accurate clocking signal. The Clock Data Recovery (CDR) elements will continue to generate a clocking signal, but this signal quickly deviates from the signal extracted from the received signal. This deviation manifests itself as a clocking signal transient. If the clocking signal transient is not suppressed, it will be propagated downstream. Typically the transient will interrupt clock recovery at downstream nodes, which in turn will propagate the timing transient. This propagation of a transient generates a cascade of element failures, with each element passing the failure-inducing transient to the next element. Because these elements often require manual intervention to re-initialize and re-synchronize with the network, it is seen as advantageous to avoid the passing of transients to other network elements. To avoid this, loss of signal conditions must be detected quickly so that a substitute clock can be used in place of the recovered clock. If the change over to the reference clock is done sufficiently quickly, the transient generated by the drifting recovered clock in the CDR is suppressed. Thus, the effect of timing transients will be minimized and will not effect downstream nodes.
When a sudden interruption to a pulse train transmitted on an optical fiber occurs, the receiver clock recovery fails and generates a transient. Without fast detection of a loss of signal, the failure will propagate downstream. Due to the feed forward nature of optical networks, the erroneously derived timing information will be propagated to downstream nodes. Many optical networking standards dictate that loss of signal must be detected and suppressed within a set number of clock cycles. However, no description of how to implement rapid transient detection is provided.
It is, therefore, desirable to provide a system for detecting the loss of signal in an optical network to prevent generation of a timing transient.